Mobile cranes are currently produced with telescopic booms or lattice booms which, due to their size, often comprise three or more axles of which in most cases at least one rear axle is actively steered.
For instance, a mobile crane is produced and distributed under the name AC 100 by Terex Demag GmbH & Co. KG located in Zweibrücken—Germany, the mobile crane comprising a chassis with five axles on which is mounted a rotatable superstructure with an extendable telescopic boom. The two front axles of this mobile crane are steered in the usual way, the first rear axle is rigid and the last two rear axles are steered actively.
One of the large mobile cranes with telescopic boom produced and distributed by Terex Demag GmbH & Co. KG/Zweibrücken—Germany is the so-called AC 200-1 TP which comprises seven axles of which the first two front axles are steered, the two in the middle are not steered when used on roads, and the other three rear axles are again actively steered. In this mobile crane, all actively steered rear axles are steerable to avoid errors caused by individual, separate rear-axle steering systems. The track-guiding front axles are usually adjusted via the steering wheel, a steering gear with a hydraulic support mechanism, and steering arms in their angular position, in other words the steering angle.
The force and steering movement is transmitted from the (several) front axle(s) to the actively steered rear axles either via mechanical arms, as it is the case in the first-mentioned smaller mobile crane, or via an electronic-hydraulic steering system, as it is the case in the last-mentioned large mobile crane AC 200-1 TP, for example. When the electronic-hydraulic rear-axle steering system is selected, the mechanical coupling to the front axles is dispensed with. This facilitates the illustration of different steering geometries. It is easily possible, for instance, to move the vehicle diagonally with all wheels being turned in the same direction. Likewise, a mechanically decoupled electronic-hydraulic rear axle steering system of this type permits a vehicle to be steered away from a wall. In that case, the steering line lies on or behind the last rear axle. Moreover, this enables one to turn corners with extremely narrow radii. In this case, the steering line lies approximately in the center of the vehicle. Electronic-hydraulic rear axle systems of this type also permit free steering, which means that the actively steerable rear axles is steered independently of the front axles, by using corresponding, associated computer systems. All these steering systems mentioned above are speed-dependent. Consequently, this results in a high maneuverability when driving slowly and in a high straight-running stability when driving fast.
Mobile cranes of the above-described type are not only used on construction sites which are closed to the public but are in particular allowed on public roads, and must therefore fulfill high safety requirements. A mobile crane of the above type must be safely controllable even in the event of an error. Usually, in order to ensure safe maneuverability even when the mobile crane is equipped with several actively steered rear axles, various fall-back levels are defined which can be achieved by means of various system concepts.
For instance, a so-called centering system is known for steering the respective rear axle to a zero-degree position. Hydraulic centering systems of this type are for example available from Mobil Elektronik, Bössingerstrasse 31-33, D-74243 Langenbrettach-Langenbeutingen. A hydraulic centering system of this type is also described in DE 102 45 618 A1, for example. As disclosed therein, each rear axle of the active rear axle steering system for a mobile crane comprises in each case two hydraulic steering cylinders and one hydraulic centering cylinder. Associated with the respective hydraulic centering cylinder are safety valves which are actuable by means of a directional control valve in such a way that they close in the event of an error so as to prevent further steering movement of the rear axles. These rear axles are in turn associated with in each case one safety valve which, in the event of a given driving situation, acts upon the centering cylinder in such a way that the associated rear axle is steerable back into the neutral position while the steering cylinders are no longer acted upon for steering displacement in a particular direction. By means of the centering cylinders, this design is meant to ensure that the associated rear axle is steerable back into the neutral position, wherein a control computer initially closes only the shut-off valves which are provided according to the invention, while the current steering position is maintained, with the result that the cornering behavior is initially maintained when the error is detected. Upon detection of a change in direction of the steering movement, the safety valve is active in such a way that the steered rear axle associated therewith is steered to the neutral position where it remains until the error is corrected.
Furthermore it shall be noted that a maximum vehicle speed may be provided as the driving situation in which the rear axles are to be steered back into the neutral position. In this case, the rear axles should be back in the neutral position as soon as a maximum vehicle speed of usually between 25 km/h and 50 km/h is reached. The active rear wheel steering system may thus be used for maneuvering during the operation of the mobile crane. When driving on roads, however, it is ensured that the steerable rear axles are in the neutral position so as to achieve a good straight-running stability at higher speeds.
Another prior-art system concept is the so-called locking system. This system ensures that in the event of an error, the respective actively steered rear axle remains in its position. A prior-art locking system, as it is for example also shown in the attached drawing explained below, is available from Mobilelektronik.
In a so-called release system, it is provided that the actively steered rear axle is configured as a trailing axle which is switched to be released in the event of an error in such a way that it follows the vehicle. This so-called release system is for example also described in the mentioned DE 102 45 618 A1.
Finally, there are combinations of the above-named systems in which, depending on the type of error, the respective actively steered rear axle is either locked or centered or released.
A combination of this type is for example described in DE 102 45 618 A1, and the electronic equipment required for a system of this type is again provided by Mobilelektronik.
All systems mentioned above are so-called fail-safe systems. A fail-operational system, which is a redundant system that continues to steer the vehicle in the event of an error, is again known from Mobilelektronik.
The active rear axle steering system according to the above described DE 102 45 618 A1 is problematic in that the hydraulic centering cylinder for in each case one actively steered rear axle and the associated hydraulic adjustment valves etc. are expensive and elaborate and therefore require a corresponding amount of space.
A device and a method of steering a vehicle with at least two steerable wheels is shown in DE 196 32 251 B4, the device comprising a steering unit for adjusting the positions of the steerable wheels in dependence on a steering setpoint signal and a unit for generating a steering behavior of the vehicle in the event of an error of the steering unit. The unit, which generates a steering behavior, is an existing braking unit which, in the event of a faulty steering system, generates selectively different braking forces acting on the wheels in dependence on the steering setpoint signal. If the error detection unit detects an error in the steering unit, the error detection unit switches off the steering unit. Simultaneously, it activates the braking unit by means of a control signal in such a way that braking forces are selectively exerted on the wheels in dependence on the steering setpoint signal, thus ensuring that the steering reactions commanded by the steering unit, which is actuable by the driver, or by an automatic vehicle guidance unit are performed.
A steering system for multi-lane vehicles is disclosed in DE 196 05 553 C1. This steering system is equipped with an emergency steering system which, in dependence on the actuation of a steering handle, is able to actuate wheel brakes on different sides of the vehicle to different extents. Likewise, as in the previously explained DE 196 32 251 B4, the sole object thereof is to steer the entire vehicle via the braking unit by applying a yaw moment.
For the purpose of a comprehensive discussion, it is necessary to mention the disclosures DE 10 2004 034 126 A1 and DE 10 2005 011 613 A1. DE 10 2004 034 126 A1 discloses a road vehicle with a steer-by-wire system. This steer-by-wire system comprises a manually actuated steering element, a sensor which detects the steering position of the steering element, and a steer-by-wire element which cooperates with steerable wheels of the vehicle. The steer-by-wire system further comprises an electronic stabilization system for the vehicle, which is actively connectable to the brakes of the vehicle, as well as a yaw rate sensor. In the event of an implausibility between the sensor of the steering element and the yaw rate sensor, a fall-back level of the steer-by-wire system is active in this system which uses the function of the electronic stabilization program to generate a steering movement of the vehicle corresponding to the steering position of the steering element and to bring the vehicle to a standstill. DE 10 2005 011 613 A1 discloses a regulation system for brake-steer-assisted parking. This regulation system is however only intended for a two-axle vehicle where it is meant to replace a four-wheel steering system with a high parking maneuverability by a more cost-effective two-wheel steering system for the front axle, combined with a brake-steering system for the rear axle.